Add a function ‘getVersion’ to get the version of a derivation

pkgs/lib/strings.nix

@@ -155,6 +155,12 @@ rec {
   # Return true iff string v1 denotes a version older than v2.
   versionOlder = v1: v2: builtins.compareVersions v2 v1 == 1;
 
+
+  # Get the version of the specified derivation, as specified in its
+  # ‘name’ attribute.
+  getVersion = drv: (builtins.parseDrvName drv.name).version;
+
+
   # Extract name with version from URL. Ask for separator which is 
   # supposed to start extension
   nameFromURL = url: sep: let